I.Clouds, circulation and climate sensitivity (Convenors: Bjorn Stevens und Sandrine Bony)Keynote presentations by Amy Clement (University of Miami, USA) and Steve Sherwood (University of New South Wales, Australia)Session description: The link between clouds and circulation are at the heart of many of climate science's enduring riddles: How rapidly and how much will the Earth warm for a given forcing? How will circulations, from the storm tracks to the tropical rain-bands, change with warming, or how different were they in the past? What role does convective aggregation play in our climate system and does it matter for climate sensitivity? To what extent do cloud feedback processes also influence climate variability? These questions are at the heart of the Grand Science Challenge on Clouds Circulation and Climate Sensitivity and motivate our session. Observational and process studies that touch on these questions in ways that link to Earth System Modelling are also very welcome.

Session description: While decadal timescales are important for adaptation decisions, predicting and attributing climate variations on decadal timescales remains challenging. Climate shows pronounced decadal variations, most recently expressed in the so-called ‘hiatus’ period. Decadal climate variability also shows a strong imprint on the global water cycle, where regional variations link to annual and decadal variability, and on a changing risk of climate extremes. This session focuses on predicting and attributing climate variations on decadal timescales. We encourage submissions on attributing climate variations and change to external forcing and climate variability, and on predicting climate on decadal timescales, both using initial conditions and using forcing and constraints based on observations. We particularly encourage submissions that touch on predicting and attributing the changing risk of extreme weather and climate events.

Session description: Modelling and reconstructing past climate change is the key to understanding Earth system dynamics at time scales beyond decades and centuries on which slow processes such as ice sheet dynamics and migration of vegetation unfold. But paleo climate science also challenges our understanding of short-term Earth system dynamics evolving under different forcing and boundary conditions. The notorious underestimation of the amplitude of tropical monsoon changes in response to different orbital forcing is one example. Syntheses of high-resolution paleo records also allow to assess the diversity of interannual to multidecadal events and their role in extreme regional conditions. Particularly interesting are tipping points, i.e., accelerated changes into a new climate and ecosystem states driven by positive feedbacks. Hence this session invites contributions from the broad spectrum of Earth system modelling of past climates and past climate change. Contributions combining model results, paleo records and explicitly modeling of past climate sensors (isotopes, ocean tracers, tree-ring, speleothems…) to test ideas related to the WCRP Grand Science Challenges are likely to have particular resonance.

Session description: The increase of atmospheric CO2, and hence climate change, is controlled by anthropogenic emissions, but also by biogeochemical processes that currently remove anthropogenic carbon dioxide from the atmosphere, helping to limit climate change. These mitigating effects may significantly reduce in a warmer climate, amplifying risk of climate change and impacts. What are the drivers of land and ocean carbon sinks? What are the origins of variability of carbon sinks and is this variability predictable? How will the carbon cycle and feedbacks operate in a warming climate and how different were they in the past? What is the potential for amplification of climate change over the 21st century via climate-carbon cycle feedbacks? How do greenhouse gases fluxes from highly vulnerable carbon reservoirs respond to changing climate? These questions are profound to the WCRP Grand Challenge on Carbon Feedbacks in the Climate System. In our session we invite contributions from modeling, observational and process studies that address these questions in ways that link to Earth System Modeling.

Session description: This session aims to connect and challenge the world of Earth System Modelling with historical, philosophical and sociological understandings of climate science and its role in society. Sociologists have recently reframed climate change as a wicked social problem rather than a matter of scientific consensus. The relation between understanding and prediction as two arguably conflicting goals in Earth System Modelling is inherently a philosophical and historical issue. Individual presentations look at the historical development of the field, its epistemic basis, its function in society and its relation to policy contexts, which shape not only international climate policy but also how scientists position their careers and research.